Normal movement is produced by an interaction of neural circuits, primarily in the spinal cord, and the musculoskeletal system. As a means of investigating this important interaction, the goal of this project will be to identify output elements of this spinal circuitry, the smallest portions of the musculoskeletal system with which spinal circuits might interact. In particular we will evaluate whether neuromuscular compartments might function as output elements by analyzing their mechanical properties. Neuromuscular compartments in the cat lateral gastrocnemius and medial gastrocnemius muscles and the human flexor carpi radialis and extensor carpi radialis longus muscles will be studied. Using a multi-axis force-moment transducer, we will determine the torques, produced either by compartment activation or by stimulation of single motor units, about three orthogonal axes at both of the joints (ankle and knee or wrist and elbow) crossed by these muscles. If compartments are output elements, then we postulate that they will exert mechanical effects, as measured by these torques, which are unique, and which are not altered when multiple compartments are activated together. We propose also that all of the motor units in a compartment would be expected to exert the same mechanical effect on the skeletal system if the compartments are output elements. The results of this study will provide important background data for other projects in this PPG as well as providing a scientific basis for the identification of output elements of spinal circuits. We anticipate that these findings will be useful for movement scientists studying the elaboration of movement by the nervous system and by rehabilitationists who have as part of their clinical role the application of therapeutic modalities involving muscle.